The invention proceeds from a method for separating the constituents of a dispersion having an electrically conducting dispersum dispersed in an insulating liquid, the dispersion being led through at least one pulsating electrical field and the dispersum being subsequently separated from the insulating liquid.
The publication by P. J. Bailes and S. K. L. Larkai, xe2x80x9cAN EXPERIMENTAL INVESTIGATION INTO THE USE OF HIGH VOLTAGE D.C. FIELDS FOR LIQUID PHASE SEPARATIONxe2x80x9d in: Trans IChemE, Vol. 59, 1981, pages 229-235, describes that in order to separate the constituents of a dispersion such as, for example, water from oil, a pulsating DC voltage with a frequency in the frequency range of 1 Hz-60 Hz is more effective than a constant one, the shape of the pulses having only a slight influence. Inter alia, the mean current intensity and the maximum voltage gradient were measured continuously over the dispersion. The high-voltage electrode was insulated using Plexiglas, thinner layer thicknesses (3 mm) yielding a larger field gradient in the dispersion than thicker ones (up to 13 mm) for the same applied voltage.
It is known from EP 0 051 463 B1 to guide a dispersion of an electrically conducting liquid in an insulating liquid, for example of water in oil, on a flow path past electrodes with a pulsating DC voltage of xe2x89xa615 kV and with a frequency in the range of 1 Hz-60 Hz, preferably in the range of 2 Hz-15 Hz. The low-voltage electrode was preferably not electrically insulated from the dispersion, while the high-voltage electrode was electrically insulated by means of Plexiglas. The maximum field strength between the electrodes on the flow path was under 1100 V/cm, preferably under 100 V/cm. The volume of the electrically conducting liquid had a proportion of  greater than 40%, preferably of 50%, of the total volume of the dispersion. After being led through the electrostatic field, the constituents of the dispersion settled in a settling tank, after which they were separated. A functional relationship is specified between the optimum frequency of the pulsating DC voltage to be applied and the thickness of the insulating layer of the high-voltage electrode.
A formula for determining the optimum frequency of the pulsating DC voltage as a function of the dielectric constant, conductivity and thickness of the insulating layers on the electrodes is specified in the publication by P. J. Bailes and S. K. L. Larkai: xe2x80x9cLIQUID PHASE SEPARATION IN PULSED D.C. FIELDSxe2x80x9d, Trans IChemE, Vol. 60, 1982, pages 115-121. Use was made in this reference of pulsating DC voltages in the region of 0.2 kV-10 kV with frequencies in the region of 0.5 Hz-60 Hz.
The invention achieves the object of further developing a method for separating the constituents of a dispersion of the type mentioned at the beginning so as to render more effective separation possible. In an exemplary method for separating the constituents of a dispersion having an electrically conducting dispersum which is dispersed in an insulating liquid, the dispersion being led through at least one pulsating electric field and the dispersum subsequently being separated from the insulating liquid, the electric conductivity "sgr"s of this dispersion is measured continuously or discontinuously, and the frequency (f) of the pulsations of the electric field is controlled as a function of the measured conductivity "sgr"s of the dispersion.
In an exemplary method, the frequency (f) of the pulsations of the electric field deviates by no more than xe2x88x9250% and +100% from a calculated frequency value fx=1/(2xc2x7xcfx80xc2x7xcfx84) where:                     τ        =                  xe2x80x83                ⁢                              ε            0                    ·                                    (                                                                    d                    s                                    ·                                      ε                    c                                                  +                                                      d                    c                                    ·                                      ε                    s                                                              )                        /                          (                                                                    d                    s                                    ·                                      σ                    c                                                  +                                                      d                    c                                    ·                                      σ                    s                                                              )                                                              c        =                  xe2x80x83                ⁢                  insulating          ⁢                      xe2x80x83                    ⁢          layer          ⁢                      xe2x80x83                    ⁢                      (                          ES2              ,                              ES2                xe2x80x2                            ,              ES3              ,                              ES3                xe2x80x2                                      )                    ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          electrodes          ⁢                      xe2x80x83                    ⁢                      (                          E2              ,                                                E2                  xe2x80x2                                ;                                                                                                                xe2x80x83                        ⁢                                          E3                ,                                  E3                  xe2x80x2                                            )                                ⁢                      xe2x80x83                    ⁢          for          ⁢                      xe2x80x83                    ⁢          applying          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢          pulsating          ⁢                      xe2x80x83                    ⁢          electric          ⁢                      xe2x80x83                    ⁢          field                ,                                                      d            c                    =                      xe2x80x83                    ⁢                      thickness            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            an            ⁢                          xe2x80x83                        ⁢            insulating            ⁢                          xe2x80x83                        ⁢            layer            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            c                          ,                                          d          s                =                  xe2x80x83                ⁢                  thickness          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          S          ⁢                      xe2x80x83                    ⁢          between          ⁢                      xe2x80x83                    ⁢          these          ⁢                      xe2x80x83                    ⁢          insulating          ⁢                      xe2x80x83                    ⁢          layers          ⁢                      xe2x80x83                    ⁢                      (                          ES2              ,                                                ES2                  xe2x80x2                                ;                                                                                      xe2x80x83                ⁢                              ES3            ,                          ES3              xe2x80x2                                )                                                                            σ                              c                .                                      ⁢                          σ              s                                =                      xe2x80x83                    ⁢                      electric            ⁢                          xe2x80x83                        ⁢            conductivity            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            c            ⁢                          xe2x80x83                        ⁢            and            ⁢                          xe2x80x83                        ⁢            S                          ,        respectively        ,                                          ε          c                ,                              ε            s                    =                      xe2x80x83                    ⁢                      dielectric            ⁢                          xe2x80x83                        ⁢            constant            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            c            ⁢                          xe2x80x83                        ⁢            and            ⁢                          xe2x80x83                        ⁢            S                          ,        respectively        ,                                          ε          0                =                  xe2x80x83                ⁢                  dielectric          ⁢                      xe2x80x83                    ⁢          constant          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          the          ⁢                      xe2x80x83                    ⁢                      vacuum            .                              
Preferably, the frequency (f) of the pulsations deviates by no more than xc2x120% from the calculated frequency value fx.
In an additional exemplary embodiment, the pulsating electric field is a field of a bipolar electric AC voltage.
In further exemplary embodiments, the dispersion is led through two pulsating electric fields, which follow one another in their flow direction. One of the two electric fields is an electric field of a pulsating DC voltage, and the other is an electric field of an AC voltage.
In yet additional exemplary embodiments, the electrically conducting droplets in the insulating liquid are electrically charged in an electric field of a charging of up to 5 kV before they are exposed to the at least one pulsating electric field of a pulsating separating voltage for the purpose of separating the constituents of the dispersion.
In a still further exemplary embodiment, the proportion of the dispersum in the dispersion is xe2x89xa640%.
In another exemplary embodiments, the insulating liquid is oil.
In still another exemplary embodiment, the frequency (f) of the pulsations of the at least one pulsating electric field is in the frequency range of  greater than 60 Hz-1 kHz.
One advantage of the invention consists in that particularly cost-effective and operationally reliable production becomes possible. Owing to the running measurement of the electric conductivity, the pulsation frequency of the electric field can be controlled at any time so as to achieve optimum separation of the dispersion constituents. Fluctuations in the conductivity of the dispersion are thereby detected in good time before poor separation on the basis of a nonoptimum pulsation frequency occurs. Consequently, it is possible to ensure continuous, optimum separation even when the nature of the initial dispersion fluctuates. It is possible as a result, for example, for the process of producing crude oil from an oil-saltwater mixture to be adapted to fluctuations in the initial water content.
An advantageous adaptation of the frequency is not limited solely to a pulsating DC voltage field. It can also be carried out for a coagulator which is operated with an AC voltage. Thus, a conventional coagulator, which is designed for operation with a 50 Hz or 60 Hz line frequency can be retrofitted by installing a conductivity measuring cell and a frequency converter so as to produce an improved separation result.
The combination of serially connected coagulator units renders it possible to optimize separately the electric charging of the water droplets and their movement in the electric field. The charging can be optimized by a stage with the highest possible charge transfer in conjunction with relatively high currents. The external electric field is then decisive, in turn, for the movement of the droplets thus charged. Said field can be kept very strong in a second stage by applying a high AC voltage via insulated electrodes, without the flow of an appreciable active current.